A PLL conventionally comprises a voltage controlled oscillator (VCO) delivering a high frequency signal, a frequency divider converting the high frequency signal into a divided frequency signal, a phase comparator producing a signal measuring a phase difference between the divided frequency signal and a reference signal, and a low-pass filter to which the measurement signal is applied and the output of which controls the VCO.
In its application to modulation, instantaneous variations of the division factor applied by the frequency divider are introduced to obtain corresponding variations in the frequency or phase of the VCO output signal.
The phase comparator can be built to activate the measurement signal during a measurement window in response to each active edge of one of the input signals of the phase comparator. The activation of the measurement signal comprises, when an active edge of the other phase comparator input signal, if necessary delayed by a predefined time, falls within the measurement window, a first pulse between the start of the measurement window and this active edge and a second pulse opposite to the first pulse between the active edge and the end of the measurement window. A drift in the phase of the loop relative to its operating point is reflected in an imbalance between the two pulses, that the low-pass filter assimilates to apply a compensation to the VCO control input.
An example of such a phase comparator is described in EP-B-0 835 550. The advantage of this comparator is that it does not present what is known as a dead zone. In ordinary phase comparators, the dead zone results from the non-zero response times of the logic gates of the comparator: phase differences smaller than these response times are not detected, so the response of the comparator presents a zero slope band (dead zone) in the vicinity of the origin. Such a dead zone affects the precision of the PLL and in practical terms prevents it from being used as a phase or frequency modulator.
The phase comparator with no dead zone described in EP-B-0 835 550 has the further advantage of presenting a dual slope response around its nominal operating point, which gives it excellent sensitivity for its application to modulation.
In this comparator, the duration of the measurement window varies according to the measured phase difference, and it depends on the delays introduced by capacitive and resistive elements of the phase comparator. The result of this is the presence of noise in the output of the comparator and therefore of a phase noise in the output of the VCO.